1. Field of the Invention
This invention relates to purification of waste water and more particularly to purification of waste water using an electroflotation process.
2. Description of the Prior Art
Various flotation methods are known for purification of waste water, which are distinguished primarily by the manner of producing the ascending stream of gas bubbles in the flotation vessel (flotation cell). Among these are compressed air, vacuum, and excess pressure flotation.
With these methods of purification by flotation the suspended particles contained in the waste water are brought to the surface of the waste water by the stream of gas bubbles and can then be removed. In the process, oily, fibrous, and flocculant materials are captured along with the suspended and floating materials. These former materials would settle out or rise to the surface only very slowly if at all.
In general, flotation, which here means the intentional bringing to the surface of suspended impurities, can be carried out by various mechanisms such as adhesion of the gas bubbles to the suspended phase and/or entrapment or absorption by air bubbles in the floc structure formed in the waste water by the addition of conventional flocculants (water soluble substances of inorganic or organic type which react with components of the waste water to form flocs).
So-called electroflotation produces a particularly fine stream of gas bubbles and has also already been suggested for purification of waste water (see, e.g., Norwegian Pat. No. 92 995). In electroflotation the waste water is brought into contact with electrodes which have an impressed electrical potential, e.g., in the range of 5-60 V. Depending on the current density, which amounts, for example, to 1-2 A/dm.sup.2 of electrode surface, the electrolytic decomposition of a small part of the waste water causes an ascending stream of very fine electrolytic gas bubbles. Such a fine stream of gas bubbles is particularly advantageous when especially fine impurities are to be removed--perhaps as a secondary purification or for special types of industrial waste water such as drilling oil emulsions--in other words, generally not the comparatively coarse particles formed by the addition to the waste water of additive solution containing flocculants.
Industrial application of electroflotation is not without various difficulties. The main, unsolvable problem associated with using electroflotation for waste water purification and keeping the cost within reason has been the formation of an oil film on the surfaces of the electrodes. When the waste water contains small amounts of emulsified oils, particularly mineral oils, an oil film forms--at least partially due to electrophoretic phenomena--on the surface of the electrode or electrodes and deactivates or "neutralizes" the electrolysis. At the relatively low voltages involved here, even a very thin oil film acts as an electrical insulator. Complete separation of oils from waste water, as a preliminary step in electroflotation perhaps, is generally made more difficult due to the fact that the water contains emulsifying surface-active agents which have gotten into the water either accidentally--perhaps as cleaning agent residues--or "intentionally" e.g., as elements of stabilized emulsions.